1. Field of the Invention
This invention relates to increasing and maintaining the hydrophilic nature of an oxidized plastic surface.
2. Background Art
Automobiles have typically been constructed of metal parts, the outer surfaces of which have been coated to provide a smooth showroom finish. The materials used to coat the metal parts comprise predominantly organic polymeric materials, or paints, such as lacquers, polyurethanes, acrylics and such, all of which provide a durable and attractive finish. More recently, automobile manufacturers have incorporated certain plastic parts into the automobile assembly process to reduce weight and improve rustproofing properties. Examples of the plastics used to form those parts include SMC (Sheet Molding Compound), polyolefins such as polypropylene (PP), polyethylene (PE, HPPE, LDPE), and olefinic blends or alloys, such as TPO (Thermoplastic Polyolefin) such as those used in the automotive industry, thermoset polyurethanes (PUR, RIM, RIMM), and thermoplastic polyurethanes (TPU), to name a few. The introduction of plastic parts into automotive assemblies has presented the automotive industry with unique challenges including the ability to maintain a uniform, constant color showroom finish on the different substrate materials. Various methods have been adopted to achieve this goal.
Typically, many of the parts making up an automotive assembly are not manufactured by the original equipment manufacturer (“OEM”) but are produced by suppliers at sites distant from where the final automobile assembly takes place. The plastic parts included in the assembly are normally molded and assembled at supplier facilities where their production and priming (in most cases) is also complex. If the plastic parts and metal parts are not subjected to a common coating or painting operation, differences in color tone and/or “look” of the coating films between the two types of materials may remain after the finish coating. It may therefore be difficult to color match both types of parts in this type of operation and more difficult to ensure the quality of the finish due to handling contamination. Therefore, many automobile manufacturers choose to precondition the entire assembly of parts, and paint the completed assembly that includes the plastic parts and the metal parts, referred to as the “body-in-white”.
The assembly plant receives the finished plastic component, where it is included in the automotive assembly, after which such components may be subjected to a variety of processes at the same time as the metal parts. These processes include steps leading up to the application of the finish coating of paint to the plastic and metal parts. More specifically, the manufacturing facility assembles the metal parts and plastic body parts into the automotive body-in-white, sends the body-in-white through several pretreatment steps and corrosion resistance processes, and lastly applies the final finish using a series of coating steps. These basic steps encompass the spraying, or dipping, of the assembly with a sequence of aqueous compositions that are recycled in a continuous manner and thereby contact newly assembled plastic and coated metal parts with aqueous compositions that have been in contact with previously treated assemblies of parts. In particular, the pretreatment operation typically comprises the steps of a power wash, a phosphate treatment, and an electrocoating process, each of which may employ an aqueous spray and/or aqueous bath dip process.
The assembly line coating of sheet metal-plastic hybrid assemblies presents problems relating to the actual coating processes and conditions. Since the physical and chemical properties of the plastic parts differ significantly from those of the metal parts, each step in the coating operation must be designed to be compatible with each type of surface. Furthermore, the end result, a showroom quality paint finish, must be practically identical for both the plastic and metal parts. The final properties of color matching, color depth, smoothness, luster, reflectivity, among others, must be substantially uniform through all visible surfaces of the finish coat assembly.
While manufacturers have been successful in designing a series of coating process steps applicable to both plastic and sheet metal parts to achieve the aforementioned goals, the automation of this process has not been altogether successful. One of the daunting problems in coating the plastic and metal parts-containing assembly has been the appearance of defects in the surface quality of the plastic parts and/or metal parts that are pretreated with one or more coatings of organic primer material. These defects manifest as surface irregularities in the final painted surfaces that detract from the acceptability of the end product. To achieve the desired factory fresh showroom finish, manufacturers find it necessary to employ time consuming manual labor to prevent these defects from appearing or to repair such defects after they appear. This of course increases the cost of the overall coating operation.
To this day, hand wiping of the assembly at various stages prior to final coating is a way, inelegant as it may seem, by which the industry has addressed the surface defect problem. Although many sources of contamination resulting in the organic surface defects are possible, a major source of contamination arises from the physical removal of uncured resin particles from the metal surfaces of assemblies undergoing the coating operation. These particles can originate with the sealers, sound deadeners, mastics and/or adhesives applied to selected portions of parts, predominately the metal parts, used in the automobile body construction, are removed therefrom by the physical forces present during the aqueous pretreatment steps, and can become suspended in the recycling aqueous compositions. On application of the aqueous composition to the sheet metal-plastic assemblies, these particles can be redeposited on the organic surfaces, or in other words transferred from the resin-treated metal surfaces to the organic surfaces. Upon curing of the final coating, the redeposited particles can cause surface irregularities or defects in the finish coat.
These particles tend to redeposit on the organic surfaces because of the hydrophobic nature of the organic surfaces. In an effort to prevent this redeposition, the automotive industry has tried to increase hydrophilicity of the surfaces by oxidizing these surfaces prior to exposing them to the aqueous compositions. This technique is bound in the theory that increasing the hydrophilicity of the organic surfaces will prevent the undesired redeposition. While this approach has a sound theoretical basis, in practice it has been found to be somewhat ineffective. This is because it has been discovered that these surfaces that have been oxidized to become hydrophilic in nature, tend to loose their hydrophilicity over time and with exposure to humidity.
It may seem to be the logical solution to oxidize these surfaces just prior to exposing them to the aqueous compositions. However, while this application may work in theory, it is not such a viable option in practice. This is because the most common approach to oxidizing the surfaces is to expose them to flame treatment. However, automobile manufactures prefer not to have flames nearby to the coating process. As such, the oxidation process usually takes place at an offsite location, where days, weeks, or months can transpire before subsequent coating occurs.
The above is just one description of a specific process employing oxidized plastic surfaces and the problems that can result. There are many other applications that require plastic surfaces to be oxidized to render them relatively more hydrophilic. For instance, plastic surfaces are usually oxidized to make them more hydrophilic so they can accept paint, ink, or adhesives better.
These oxidized surfaces face the same problems as those described above. They tend to revert back to a more hydrophobic state over time. This causes timing issues for processes employing oxidized plastic surfaces.
Polyphenol compounds have long been used in the metal finishing art for the treating of metal surfaces to provide a coating on the metal surfaces which is effective in enhancing the corrosion resistance and paint adhesion characteristics of the metal surfaces. Such polyphenol compounds are disclosed in U.S. Pat. Nos. 4,433,015, 4,517,028, 4,963,596, 4,970,264 and 5,039,770, which are incorporated herein by reference. These compounds have not been used before to treat oxidized organic surfaces.
Accordingly, it would be desirable to provide a method of increasing the hydrophilicity of organic surfaces for relatively extended periods of time. It would also be desirable to provide a method of improving adhesion of paint, ink, and adhesives. It would also be further desirable to provide a method of preventing particle redeposition onto organic surfaces which is not time dependent.